1. Field of the Invention
This invention relates to a method of compressing image signals, and an apparatus for carrying out the method. This invention particularly relates to a method of and apparatus for compressing image signals wherein digital signals of a radiation image of the human body are optimally entropy-encoded when radiation image information obtained by reading out the radiation image of the human body is stored as digital signals in a storage medium.
2. Description of the Prior Art
When image signals or the like are stored in memory media such as magnetic tapes for later use, it is desired to compress the image signals for the purpose of storing large amounts of signals in as small storage space as possible. Thus various signal compression systems have heretofore been proposed and employed. One example of the signal compression systems is the entropy encoding system wherein digital signals are encoded by use of values on the order of entropy. In an entropy encoding system, a Huffman code, a Shannon-Fano code, or the like is used.
In the aforesaid entropy encoding system, a code consisting of a small number of bits (for example, the Huffman code) is assigned to signals exhibiting high appearance probability among the signals of various values which arise, and a code consisting of a comparatively large number of bits is assigned to signals exhibiting low appearance probability. As a result, the total number of bits used for encoding is reduced to compress the amount of signals.
For entropy encoding as described above, it is necessary to prepare an encoding table suitable for the kind of signal (the information) to be encoded. That is, effective signal compression cannot be realized by the entropy encoding unless the encoding table is suitable for the kind of signal. Specifically, in order to efficiently compress, for example, image signals by entropy encoding, it is necessary to create an optimal encoding table on the basis of a statistical analysis of each image and to carry out signal compression by use of the created table.
Therefore, in the entropy encoding system as described above, it is necessary to prepare a separate table for entropy encoding for each image, which will realize effective data compression. However, an extra time and labor are required for the preparation of the tables. Accordingly, the entropy encoding system is disadvantageous in practical use.
The aforesaid entropy encoding system is thus desired to be improved particularly when it is necessary to process and store large numbers of images.